How to make three processes work in C/C++ - c++

Question

I have to make three processes A, B and C that use shared memory. A and B write 100 integers in the shared memory, and C reads them and writes them to a binary file. That is what I made, but it doesn't work properly. I include <stdio.h>, <math.h>, <fcntl.h> and <time.h>. How to make it work?
struct sync
{
int n;
int lock;
int generated;
char process;
} *b;
int testandset(int* lockPtr)
{
int oldValue = *lockPtr;
return 0 != oldValue;
}
int main()
{
struct sync buff;
int pid, ppid, fp, i;
srand(time(NULL));
b = (struct sync*)malloc(666);
b->n = 0;
b->lock = 0;
b->generated = 0;
i = 0;
printf("Generating numbers\n");
pid = fork();
if (0 == pid)
{
while (100 >= b->generated)
{
while (testandset(&(b->lock)))
{
}
buff.n = rand() % 1001;
buff.process = 'A';
fp = open("db", O_RDWR | O_APPEND);
if (-1 == fp)
fp = open("db", O_CREAT);
write(fp, &buff, sizeof(struct sync));
close(fp);
b->generated++;
b->lock = 0;
}
}
if (0 < pid)
{
ppid = fork();
if (0 == ppid)
{
while (100 >= b->generated)
{
while (testandset(&(b->lock)))
{
}
buff.n = rand() % 1001;
buff.process = 'B';
printf("No: %d %d \n", ++i, buff.n);
fp = open( "db", O_RDWR | O_APPEND );
if (-1 == fp)
fp = open("db", O_CREAT);
write(fp, &buff, sizeof(struct sync));
close(fp);
b->generated++;
b->lock = 0;
}
}
if (0 < ppid)
{
wait();
i = 0;
fp = open("db", O_RDONLY, 0755);
printf("Reading from file\n");
while (read(fp, &buff, sizeof(struct sync)))
{
if ('A' == buff.process)
i++;
}
close(fp);
int vals[i];
i = 0;
fp = open("db", O_RDONLY, 0666);
while (read(fp, &buff, sizeof(struct sync)))
{
if ('A' == buff.process)
vals[i++] = buff.n;
}
close(fp);
fp = open("db", O_RDONLY, 0455);
int i;
for(i = 0; i < i; i++)
write((const void*) &vals[i],sizeof(int),1,fp);
}
wait();
}
return 0;
}

Answer 1

Files are not really reliable for mutliprocess sharing information (and editing at the same time), You'd rather use a real database in transaction mode or use IPC http://www.cs.cf.ac.uk/Dave/C/node27.html. Or maybe re-design and use threads and mutex.

Answer 2

There are many issues here, but I will concentrate on testandset(). Firstly, it's misnamed, it doesn't set anything, nor do you set b->lock anywhere. What's it for?
It seems to me that you might be trying to use it for some sort of locking between processes, but it won't work. When you fork, the new child effectively gets a complete copy of the parent's address space and changes to variables in the parent will not be seen in the child, and vice versa. As Maresh says, you need to look at using inter process communication.